1. Field of the Invention
The present invention relates to a wideband amplifier for use, for example, in an AM radio receiver.
2. Description of the Related Art
When integrating a radio receiver into a one-chip IC arrangement, if the intermediate-frequency filter is formed of a ceramic filter or the like, it becomes impossible to incorporate the intermediate-frequency filter into the IC.
Accordingly, it is considered as a solution to form the intermediate-frequency filter of an active filter using resistors, capacitors, and operational amplifiers. In such case, if the intermediate frequency f.sub.i is set to its standard value, 450 kHz, satisfactory results cannot be obtained because, then, the area occupied by the active filter on the semiconductor pellet becomes too great.
Accordingly, it is further considered to reduce the intermediate frequency f.sub.i to a value sufficiently lower than the frequency band of the received signal, to 55 kHz, for example.
FIG. 3 is a diagram showing an example of such IC or radio receiver. Referring to FIG. 3, the portion 10 enclosed by a chain line indicates an IC for the one-chip IC AM receiver and T1 to T8 denote its external terminal pins, of which the pin T3 is a power supply pin and the pin T4 is a grounding terminal pin.
The parts located outside the chain line are parts or circuits externally connected to the IC, of which reference numeral 1 denotes an antenna tuner and 2 denotes a resonator for local oscillation. The tuner 1 is formed of a bar antenna (antenna tuning coil) L1 and a variable capacitor VC1 and the resonator 2 is formed of a local oscillation coil L2 and a variable capacitor VC2 associated with the variable capacitor VC1.
Further, SW denotes a power supply switch, BATT denotes a power supply battery, for example, of 3 V, VR denotes a variable resistor for volume control, and SP denotes a speaker.
By the antenna tuner 1, a broadcast wave signal S.sub.r with a frequency f.sub.r EQU S.sub.r =Er.multidot.sin .omega..sub.r t EQU .omega..sub.r =2.pi.f.sub.r
is selectively acquired. Since what are relevant to signal processes described hereinafter are only the relative amplitude and phase between these signals, the initial phase is neglected in the above expression and in the following description.
The signal S.sub.r is supplied to a high-frequency amplifier 11 through the terminal pin T1 of the IC 10, and the signal S.sub.r from the amplifier 11 is supplied to a first and a second mixer 12A and 12B.
To the resonator 2, a local oscillator 13 is connected through the terminal pin T2, and therein, a local oscillation signal S.sub.o is generated. At this time, the oscillation frequency of the oscillation signal S.sub.o is set to 2f.sub.o such that EQU 2f.sub.o =(f.sub.r +f.sub.i).times.2,
where f.sub.i is an intermediate frequency, f.sub.i =55 kHz.
The oscillation signal S.sub.o is supplied to a frequency divider (counter) 14 and divided into local signals S.sub.oa and S.sub.ob with a half the original frequency and a phase difference of 90.degree. therebetween. More specifically, the oscillation signal S.sub.o is divided for frequency into S.sub.oa and S.sub.ob such that EQU S.sub.oa =E.sub.o .multidot.cos .omega..sub.o t, EQU S.sub.ob =E.sub.o .multidot.sin .omega..sub.o t,
where EQU .omega..sub.o =2.pi.f.sub.o.
These signals S.sub.oa and S.sub.ob are supplied to the mixers 12A and 12B, respectively, to be multiplied by the signal S.sub.r, and the following signals S.sub.ia and S.sub.ib are taken out from the mixers 12A and 12B ##EQU1##
As described later, the signal components with the angular frequency (.omega..sub.r -.omega..sub.o) out of the above signals S.sub.ia and S.sub.ib are used as the intermediate-frequency signals and the signal components with the angular frequency (.omega..sub.r +.omega..sub.o) are eliminated. Hence, neglecting, for simplicity, the signal components with the angular frequency (.omega..sub.r +.omega..sub.o) in the above expressions, we obtain EQU S.sub.ia =.alpha..multidot.sin(.omega..sub.r -.omega..sub.o)t, EQU S.sub.ib =.alpha..multidot.cos(.omega..sub.r -.omega..sub.o)t.
An image signal S.sub.m, at this time, is expressed as EQU S.sub.m =E.sub.m .multidot.sin .omega..sub.m t,
where EQU .omega..sub.m =.omega..sub.o +.omega..sub.i, EQU .omega..sub.i =2.pi.f.sub.i.
Hence, if the image signal S.sub.m is included in the broadcast wave signal S.sub.r from the tuner 1, then the signals S.sub.ia and S.sub.ib' become EQU S.sub.ia =.alpha..multidot.sin(.omega..sub.r -.omega..sub.o)t+.beta..multidot.sin (.omega..sub.m -.omega..sub.o)t, EQU S.sub.ib =.alpha..multidot.cos(.omega..sub.r -.omega..sub.o)t+.beta..multidot.cos(.omega..sub.m -.omega..sub.o)t,
where .beta.=E.sub.m .multidot.E.sub.o /2. Further, since EQU .omega..sub.r &lt;.omega..sub.o &lt;.omega..sub.m,
the above expressions become ##EQU2##
These signals S.sub.ia and S.sub.ib are supplied to phase shifters 15A and 15B. The phase shifters 15A and 15B are formed, for example, of active filters using capacitors, resistors, and operational amplifiers. The signal S.sub.ia is shifted for phase in the phase shifter 15A by a value .phi. and the signal S.sub.ib is shifted for phase in the phase shifter 15B by a value (.phi.+90.degree.), and thereby, the two input signals S.sub.ia and S.sub.ib are shifted for phase so as to have the phase difference 90.degree..+-.1.degree. therebetween, within the frequency range of 55 kHz.+-.10 kHz.
Thus, the signal S.sub.ib is arranged to lead the signal S.sub.ia by 90.degree. after phase shifting by the phase shifters 15A and 15B, and these signals become ##EQU3## These signals S.sub.ia and S.sub.ib are supplied to an adder 16 to be added together. From the adder 16 is obtained a signal S.sub.i expressed as ##EQU4## the signal S.sub.i is the desired intermediate-frequency signal. Further, it is known that, even if an image signal S.sub.m is included in the broadcast wave signal S.sub.r from the tuner 1, the intermediate-frequency signal S.sub.i does not include signal components due to the image signal S.sub.m because such signal components cancel each other.
Thus, the intermediate-frequency signal S.sub.i (and signal components with the angular frequency (.omega..sub.r +.omega..sub.o) and others) converted from the broadcast wave signal S.sub.r can be obtained from the adder 16.
The intermediate-frequency signal S.sub.i is supplied to a bandpass filter 17 for intermediate-frequency filtering. The bandpass filter 17 is formed, for example, of a biquad active filter using capacitors, resistors, and operational amplifiers and its passband is set to be 55 kHz.+-.3 kHz. Thus, unnecessary signal components are attenuated by the bandpass filter 17 and only the intermediate-frequency signal S.sub.i is taken out.
The thus obtained intermediate-frequency signal S.sub.i is supplied to an AM detector 22 through an amplifier 21, and thereby, an audio signal S.sub.s (and a DC component V22 corresponding to the level of the intermediate-frequency signal S.sub.i) is taken out and this audio signal S.sub.s is supplied to an audio amplifier 23 with differential inputs. The signal S.sub.s from the amplifier 23 is supplied to the speaker SP through the pin T8 and a capacitor C5.
The signal S.sub.ib from the mixer 12B is supplied to an AGC voltage generator 18 and, therein, an AGC voltage is generated. This AGC voltage is supplied to the amplifier 11 as the control signal of its gain and, therein, an AGC operation is performed on the signals S.sub.ia and S.sub.ib. In this case, the generator 18 is connected to a capacitor C3 for smoothing the AGC voltage through the pin T5. The AGC voltage is further supplied as the reference voltage to each of operational amplifiers constituting the phase shifters 15A and 15B and the bandpass filter 17.
The detected output from the detector 22 is supplied to an AGC voltage generator 24 to obtain an AGC voltage, and this AGC voltage is supplied to the amplifiers 11 and 21 as the control signal of their gains and, therein, AGC operations are performed on the signals S.sub.ia, S.sub.ib, and S.sub.i.
In this case, the generator 24 is connected to a capacitor C4 through the pin T6, and a low-pass filter is formed with this capacitor C4, and thereby, the DC voltage V22 is extracted from the detected output, and from this DC voltage V22 is generated the AGC voltage. The DC voltage V22 is supplied to the differential input of the amplifier 23, whereby the DC component V22 supplied, together with the audio signal S.sub.s, from the detector 22 to the amplifier 23 is equivalently canceled.
Further, the amplifier 23 is connected with the variable resistor VR through the pin T7. The gain of the amplifier 23 is controlled according to the resistance value of the variable resistor VR and, thus, the volume control is achieved with this variable resistor VR.
The capacitor C6 is for bypassing signal components other than the audio signal S.sub.s.
In the present example, since the intermediate frequency f.sub.i is sufficiently lower than the general intermediate frequency or receive band, the area occupied by the bandpass filter (intermediate-frequency filter) 17 for each step becomes larger but the number of steps for obtaining a required selectivity characteristic can be decreased. Accordingly, the area occupied by the whole of the bandpass filter 17 can be made smaller and hence it can be integrated into the IC arrangement.
When the intermediate frequency f.sub.i is low, the image characteristic generally becomes worse, but since the image signal S.sub.m is removed by 12A, 15A, and 16, the image characteristic is prevented from becoming worse.
Further, since the phase shifters 15A and 15B as well as the bandpass filter 17 are formed of active filters, there is a limit in the signal level which can be handled by each of the circuits 15A, 15B, and 17. However, since AGC is applied to the amplifier 12, the phase shifters 15A and 15B and the bandpass filter 17 can be prevented from being supplied with excessive inputs. An arrangement of AM/FM receiver with the described idea applied thereto is already disclosed in Japanese Laid-open Patent Publication No. 1-273432 (to which U.S. Pat. No. 5,020,147 corresponds).
When a one-chip IC is arranged with the above idea applied thereto and a radio receiver is integrated into such an arrangement, the frequency band of the audio amplifier 23 extends to the receive frequency band of the receiver. Accordingly, a high-frequency feedback loop passing through the bar antenna L1.fwdarw.receiving circuit system of the IC 10.fwdarw.output terminal pin T8.fwdarw.bar antenna L1 is formed, and therefore, when the output of the amplifier 23 is made greater (when the gain of the amplifier 23 is increased), a troublesome oscillating state is produced in the high-frequency band.
To avoid such trouble, such a method as follows can be considered to cut off the feedback loop:
1. To connect a capacitor C6 between the pin T8 and ground;
2. To increase the distance between the bar antenna L1 and the output terminal pin T8;
3. To insert a high-frequency choke coil between the output terminal pin T8 and the speaker SP; or
4. To insert a high-frequency choke coil between the power supply terminal pin T3 and the switch SW.
However, when the method of the item number 1 is used, it is required to make the impedance of the capacitor C6 sufficiently smaller than the impedance of the speaker SP at the frequency causing the oscillation. To meet this requirement, the capacity of the capacitor C6 must be great. Then, it becomes impossible to integrate the capacitor C6 into the IC 10, and it becomes necessary to attach the capacitor C6 externally to the IC 10, which leads an increase in the number of parts used.
When the method of the item number 2 is used, though the number of the parts is not increased, the miniaturization of the receiver becomes unachievable. Further, when the method of the item number 3 or 4 is used, the coil cannot be integrated into the IC and, hence, the number of parts increases, the same as in the case where the method of the item number 1 is used.
Thus, it has been difficult to obtain a miniaturized high-performance receiver.